It has been recognized that due to environmental concerns and newly enacted rules and regulations, saleable petroleum products must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from liquid hydrocarbons that are used in transportation fuels, such as gasoline and diesel. For example, ultra low sulfur diesel (ULSD) requires typically less than about 10 wppm sulfur.
Hydroprocessing is a process that contacts a selected feedstock and hydrogen-containing gas with suitable catalyst(s) in a reaction vessel under conditions of elevated temperature and pressure. Hydrocracking refers to a process in which hydrocarbons crack in the presence of hydrogen and catalyst to lower molecular weight hydrocarbons. Hydrocracking is a process used to crack hydrocarbon feeds such as vacuum gas oil (VGO) to diesel including kerosene and gasoline motor fuels. Hydrotreating is a type of hydroprocessing active for the removal of heteroatoms, such as sulfur and nitrogen, and saturating unsaturated compounds in the hydrocarbon feedstock.
Hydrotreating and hydrocracking converts sulfur on hydrocarbons to hydrogen sulfide and nitrogen on hydrocarbons to ammonia. Ammonia is a catalyst poison for hydroprocessing catalyst such as hydrocracking catalyst and saturation catalyst, particularly, noble metal saturation catalyst. Hydrogen sulfide and ammonia gases are stripped from liquid hydrocarbon streams to prepare them for further catalytic processing and to provide fuel products with low sulfur.
At higher pressures, such as 12.4 MPa (1800 psig) to 17.2 MPa (2500 psig), hydrotreating can also saturate aromatic compounds to increase the cetane number of diesel produced from a hydrocarbonaceous feed or to render it more susceptible to hydrocracking However, at lower pressures hydrotreating catalyst is less effective in saturating aromatics. High pressure processing is more expensive on capital and operational bases because it requires more robust metallurgy and compression systems.
There is a continuing need, therefore, for improved methods to produce diesel product with lower sulfur content and higher cetane value at lower cost.